1. Field of the Invention
The present invention relates to a form measuring apparatus and a form measurement method.
2. Description of Related Art
Nowadays, form measuring apparatuses measuring a surface form of an object are in widespread use. One example of such a form measuring apparatus is a form measuring apparatus measuring a form of a measured object by displacing a stylus head provided to a probe such that the stylus head scans a surface of a measured object (Japanese Patent Laid-open Publication No. 2008-241420).
This form measuring apparatus converts design values based on CAD data or the like (for example, Non-Uniform Rational B-Spline data, hereafter referred to as NURBS data) into parametric cubic curves (hereafter referred to as PCC curves). The form measuring apparatus calculates a speed curve from divided PCC curves and calculates a displacement speed of the probe based on the calculation results. In addition, the form measuring apparatus displaces the stylus head so as to scan the surface of the measured object by displacing the probe based on the calculated displacement speed. This measurement method is referred to as nominal scanning measurement (see, e.g., Japanese Patent Laid-open Publication No. 2008-241420).
Another measurement method is known for a form measuring apparatus, in which probe displacement is controlled without using design data. In this measurement method, an amount of depression of the probe toward the measured object (i.e., the distance the stylus is withdrawn into the probe when the stylus is depressed against the measured object) is detected. Then, the probe is displaced so as to scan the surface of the measured object such that the amount of depression matches a predetermined reference amount of depression. This measurement method is referred to as autonomous scanning measurement (see, e.g., Japanese Patent Laid-open Publication No. 2005-345123).
However, the inventors of the present invention have discovered an issue, discussed below, with respect to the above-noted technique. In nominal scanning measurement (Japanese Patent Laid-open Publication No. 2008-241420), the stylus head is displaced along a measurement path defined in advance based on design values. Therefore, high-speed form measurement can be carried out. However, this does not take into consideration changes in the amount of depression of the probe toward the measured object. Therefore, in a case where an indentation not present in the design exists on the surface of the measured object, when the stylus head displaces over the unexpected indentation, the amount of depression of the stylus head drops (i.e., the distance the stylus withdraws into the probe when the stylus head contacts the measured object is reduced). Further, in a case where a projection not present in the design exists on the surface of the measured object, when the stylus head displaces over the unexpected projection, the amount of depression of the stylus head increases (i.e., the distance the stylus withdraws into the probe when the stylus head contacts the measured object is increased). As a result, when the amount of depression of the stylus head exceeds an allowed range, measurement data cannot be obtained and form measurement is interrupted.
The present invention was conceived in light of the circumstances above, and when performing form measurement with a form measuring apparatus, the present invention continues the form measurement even in a case where a measurement error occurs in which an amount of depression of a stylus head exceeds an allowed range.